1. Field of the Invention
The present invention relates to a radiography apparatus and a radiography method for acquiring image data with an imaging unit, and particularly to a radiography apparatus and a radiography method with a function for evaluating a degradation over time in quality of an image acquired with an imaging unit.
2. Description of the Related Art
In a known radiography apparatus, a radiation source emits a radiation beam onto an object to be analyzed, such as a medical patient. The radiation beam that has passed through the patient then enters a screen film cassette, a film autochanger, a computed radiography (CR) system, a flat panel detector (FPD), or the like for imaging. Radiographs are also rapidly becoming digital, just like other consumer products.
Recently, a technology has been developed for acquiring digital images by using an image-receiving unit called an FPD, which is a photoelectric converting unit having pixels such as small imaging elements or switching devices arranged in an array. Some advantages associated with the utilization of such a photoelectric converting unit are as follows. First, since images acquired are digital data, image processing, such as correction for undesirable radiographic conditions and enhancement of a region of interest, is easy. Furthermore, patients in a remote hospital without a specialist can be diagnosed by a specialist in a separate large hospital with an image-communicating unit via a large-capacity communication line. In addition, the storage space required for digital images is much smaller than the storage space required for images on film because digital image data can be stored on, for example, a magneto-optical disk. Yet another advantage is that it is easier to search for historical images stored in digital format, so that reference images can be presented more easily than when images on film are searched for, as described in Japanese Patent No. 3413084.
A typical radiography apparatus is subject to degradation in image quality as it repeats radiography over time, that is, as its operation time accumulates. Unfortunately, an objective invariance test for evaluating degradation in image quality over time has not yet been conducted in known radiography apparatuses.